The vacuum-induced, countergravity, casting process is useful in the making of thin-walled near-net-shape castings and involves: sealing a bottom-gated mold, having a gas-permeable upper portion, to the mouth of a vacuum chamber such that the chamber confronts the upper portion; immersing the underside of the mold in an underlying melt; and evacuating the chamber to draw melt up into the mold through one or more of the gates in the underside thereof. Such a process is shown in Chandley et al. U.S. Pat. No. 4,340,108 wherein the mold comprises a resin-bonded-sand shell having an upper cope portion sealingly bonded (e.g., glued) to a lower drag portion and the vacuum chamber sealed atop the cope such that the parting line between the cope and drag lies outside the vacuum chamber. Almond U.S. Pat. No. 4,632,171, assigned to the assignee of the present invention, is an improvement on Chandley et al. U.S. Pat. No. 4,340,108 and seals the mold to the vacuum chamber atop the drag such that the parting line between the cope and drag falls within the vacuum chamber. In such processes, it is desirable to immerse the drag as far as possible into the melt to prevent invasion of the mold cavity by air being sucked through any portions of the porous drag which might be above the melt surface. Hence, in both the Chandley et al and Almond cases, the parting line between the cope and the drag is brought into close proximity to the surface of the underlying melt during casting. Moreover, in Almond's case, the seal between the vacuum chamber and the mold is also brought into close proximity to the surface of the underlying melt during immersion.
It is undesirable to have melt contact either the parting line between the cope and the drag or the joint between the vacuum chamber and the mold. In this regard, melt at the parting line: (1) can cause gasification of any glue therein which gases are drawn into the mold cavity and can be trapped in the casting; and (2) can be sucked into the mold cavity at the parting line thereby ruining the casting. Moreover, hot melt at the chamber-mold joint can be sucked directly into the vacuum chamber causing ignition of the gases therein and result in destruction of the mold, casting and possibly the chamber itself.
It is not always possible to control precisely the depth that the mold is immersed into the underlying melt. Hence there is a risk that the cope-to-drag parting line and/or the chamber-to-mold joint may accidentally be contacted by the hot melt unless some technique can be devised for isolating them from the melt under all circumstances.
It is an object of the present invention to provide improved apparatus for the vacuum-induced, countergravity casting of metal into porous, bottom-gated molds wherein the cope-to-drag parting line and/or the chamber-to-mold joint is isolated from the melt over a wide range of immersion depths. This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.